A typical face-type shaft seal used in a centrifugal pump comprises an axially movable, rotating seal ring carried on the pump shaft and a seated, non-rotating seal ring which also surrounds the shaft. In normal operation the seal faces of the rings are maintained in engagement by a coil spring that surrounds the shaft and urges the rotating ring toward the seated ring. A flexible sleeve having one end fixed to the shaft and its other end to the rotating ring completes the fluid seal. The flexible sleeve is typically in the form of a bellows that will accommodate axial movement of the rotating ring toward the seated ring.
Centrifugal pumps may be exposed to operational conditions varying from no flow or maximum pressure to a maximum flow, low pressure condition. On either side of the best efficiency point on the pump curve, pressure variations within the pump and seal cavity impose a radial load on the impeller which will produce deflection of the shaft. The resultant shaft deflection and pressure changes tend to separate the seal rings of the shaft seal as described above. When separation occurs, abrasive materials handled by the pump can enter the space between the seal rings and cause extreme wear and total seal failure in a short time. Misalignment of the seal faces caused by deflections of the pump shaft also can result in only partial contact of the seal ring faces and uneven wear. The effectiveness and service life of a shaft seal in a centrifugal pump therefore depends upon the ability of the rotating seal ring to float and maintain contact with the seated ring when shaft deflections and pressure changes occur.
In pumping applications involving slurries or suspensions of coherent materials i.e., materials having cementation characteristics, the solid particles can accumulate within the rotating seal assembly and pack around the flexible sealing sleeve to fill the bellows convolutions. The packed material can ultimately harden into a solid, cemented mass that impairs contraction and elongation of the bellows sealing sleeve which is necessary to provide floating movement of the rotating seal ring. Under these conditions, the floating function of the rotating seal ring may be completely destroyed and the ring will be unable to maintain face contact with the stationary seat ring.
In one prior art arrangement, a protective boot or sleeve has been provided around the coil spring. The difficulty with this prior arrangement is that an increase of pressure in the seal cavity can effectively shorten the protective boot, so as to reduce the spring preload needed to keep the seal rings in engagement. If the spring loading is increased in an attempt to compensate for the counterbalancing effect of shortening the protective boot, the seal rings may be forced together so tightly under normal operating conditions that undue wear results.